U.S. patent application number 10/727671 was filed with the patent office on 2004-07-22 for variable optical attenuator.
This patent application is currently assigned to Fujitsu Limited. Invention is credited to Kishida, Toshiya, Kunikane, Tatsuro, Yamane, Takashi.
Application Number | 20040141710 10/727671 |
Document ID | / |
Family ID | 32709221 |
Filed Date | 2004-07-22 |
United States Patent
Application |
20040141710 |
Kind Code |
A1 |
Kishida, Toshiya ; et
al. |
July 22, 2004 |
Variable optical attenuator
Abstract
A variable optical attenuator is constituted of input/output
optical systems, a birefringent member provided at output sides of
the input/output optical systems, a liquid-crystal member capable
of individually varying polarizing states of input beams exiting
the birefringent member, and a reflection member which reflects
light passing through the liquid-crystal member, to thereby cause
the light to return to an output lens of the input/output optical
systems by way of the liquid-crystal member and the birefringent
member. Thus, there can be provided a variable optical attenuator
which is more compact and less expensive than a related-art
variable optical attenuator.
Inventors: |
Kishida, Toshiya; (Kawasaki,
JP) ; Yamane, Takashi; (Kawasaki, JP) ;
Kunikane, Tatsuro; (Kawasaki, JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
SUITE 700
1201 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Assignee: |
Fujitsu Limited
Kawasaki
JP
|
Family ID: |
32709221 |
Appl. No.: |
10/727671 |
Filed: |
December 5, 2003 |
Current U.S.
Class: |
385/140 |
Current CPC
Class: |
G02F 1/13 20130101; G02B
6/266 20130101; G02F 2203/48 20130101 |
Class at
Publication: |
385/140 |
International
Class: |
G02B 006/26 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 20, 2003 |
JP |
2003-011424 |
Claims
1. A variable optical attenuator comprising: an input/output
optical system to which are connected a plurality of input optical
fibers and a plurality of output optical fibers and which has a
plurality of input lenses for taking beams having entered by way of
said input optical fibers as input beams and a plurality of output
lenses for gathering output beams to be coupled to said output
optical fibers, to thereby couple said output beams to said output
optical fibers; a birefringent device provided on an output side of
said input/output optical system; a liquid crystal device capable
of changing polarizing states of said input beams exiting said
birefringent device; and a reflection device which reflects beams
passing through said liquid-crystal device so that the beams return
to said output lens of said input/output optical system by way of
said liquid-crystal device and said birefringent device.
2. The variable optical attenuator according to claim 1, wherein
said input/output optical system, said birefringent device, said
liquid-crystal device, and said reflection device are integrated
together.
3. The variable optical attenuator according to claim 2, wherein
said input/output optical system comprises a fiber array block, in
which a plurality of said input optical fibers are arranged and
connected in the form of an array and a plurality of said output
optical fibers are arranged and connected in the form of an array
in the same direction as that in which the input optical fibers are
arranged; and a lens array block, in which a plurality of said
input lenses are arranged in the form of an array in accordance
with the arrangement of said input optical fibers in said input
array fiber block and in which a plurality of said output lenses
are arranged in the form of an array in accordance with the
arrangement of said output optical fibers in said output array
fiber block.
4. The variable optical attenuator according to claim 3, wherein a
pitch between said input optical fibers and a pitch between said
output optical fibers are set so as to become greater than a pitch
between said input lenses and a pitch between said output
lenses.
5. The variable optical attenuator according to claim 3, wherein
said input/output optical system has a prism unit which is
interposed between said fiber array block and said lens array block
and which reflects a portion of incident light in a direction
crossing the direction of an optical axis; and a light-receiving
unit for monitoring input and output light which receives the light
reflected from said prism unit.
6. The variable optical attenuator according to claim 5, wherein
said light-receiving unit is formed from a photodiode array, in
which a plurality of photodiodes, each photodiode having a P
electrode on one surface thereof and an N electrode on the other
surface thereof, are arranged in an array pattern on a conductive
transparent substrate such that said other surfaces come into
contact with said transparent substrate; and wherein a common
terminal of said N electrodes of said respective photodiodes are
provided on said transparent substrate.
7. The variable optical attenuator according to claim 5, wherein
said light-receiving unit is formed from a photodiode array, in
which a plurality of photodiodes, each having a P electrode on one
surface thereof and an N electrode formed around said P electrodes,
are arranged in the form of an array on a transparent
substrate.
8. the variable optical attenuator according to claim 1, wherein
said input/output optical system comprises a fiber array block, in
which a plurality of said input optical fibers are arranged and
connected in the form of an array and a plurality of said output
optical fibers are arranged and connected in the form of an array
in the same direction as that in which the input optical fibers are
arranged; and a lens array block, in which a plurality of said
input lenses are arranged in the form of an array in accordance
with the arrangement of said input optical fibers in said input
array fiber block and in which a plurality of said output lenses
are arranged in the form of an array in accordance with the
arrangement of said output optical fibers in said output array
fiber block.
9. The variable optical attenuator according to claim 8, wherein a
pitch between said input optical fibers and said output optical
fibers is set so as to be greater than a pitch between said input
lenses and a pitch between said output lenses.
10. The variable optical attenuator according to claim 8, wherein
said input/output optical system has a prism unit which is
interposed between said fiber array block and said lens array block
and which reflects a portion of incident light in a direction
crossing the direction of an optical axis; and a light-receiving
unit for monitoring input and output light which receives the light
reflected from said prism unit.
11. The variable optical attenuator according to claim 10, wherein
said light-receiving unit is formed from a photodiode array, in
which a plurality of photodiodes, each photodiode having a P
electrode on one surface thereof and an N electrode on the other
surface thereof, are arranged in an array pattern on a conductive
transparent substrate such that said other surfaces come into
contact with said transparent substrate; and wherein a common
terminal of said N electrodes of said respective photodiodes is
provided on said transparent substrate.
12. The variable optical attenuator according to claim 10, wherein
said light-receiving unit is formed from a photodiode array, in
which a plurality of photodiodes, each having a P electrode on one
surface thereof and an N electrode formed around said P electrode,
are arranged in the form of an array on a transparent
substrate.
13. The variable optical attenuator according to claim 1, wherein
said reflection device is formed from a coupler film which permits
transmission of a portion of the light exiting the liquid-crystal
device; and an input light monitor light-receiving unit for
receiving the light having passed through said coupler film is
provided on the surface of said coupler film.
14. The variable optical attenuator according to claim 13, wherein
said light-receiving unit is formed from a photodiode array, in
which a plurality of photodiodes, each photodiode having a P
electrode on one surface thereof and an N electrode on the other
surface thereof, are arranged in an array pattern on a conductive
transparent substrate such that said other surfaces come into
contact with said transparent substrate; and wherein a common
terminal of said N electrodes of said respective photodiodes is
provided on said transparent substrate.
15. The variable optical attenuator according to claim 13, wherein
said light-receiving unit is formed from a photodiode array, in
which a plurality of photodiodes, each having a P electrode on one
surface thereof and an N electrode formed around said P electrode,
are arranged in the form of an array on a transparent
substrate.
16. The variable optical attenuator according to claim 1, wherein
said input/output optical system is provided with an output light
monitor light-receiving unit for receiving the light that is not
coupled to said output optical fiber as a result of a variation in
the polarizing state of said liquid-crystal device from among the
beams reflected from said reflection device.
17. The variable optical attenuator according to claim 16, wherein
said light-receiving unit is formed from a photodiode array, in
which a plurality of photodiodes, each photodiode having a P
electrode on one surface thereof and an N electrode on the other
surface thereof, are arranged in an array pattern on a conductive
transparent substrate such that said other surfaces come into
contact with said transparent substrate; and wherein a common
terminal of said N electrodes of said respective photodiodes is
provided on said transparent substrate.
18. The variable optical attenuator according to claim 12, wherein
said light-receiving unit is formed from a photodiode array, in
which a plurality of photodiodes, each having a P electrode on one
surface thereof and an N electrode formed around said P electrode,
are arranged in the form of an array on a transparent
substrate.
19. The variable optical attenuator according to claim 1, wherein
said liquid-crystal device has a plurality of sets, each set
comprising liquid crystal and electrodes to be used for applying an
electric field to said liquid crystal, for controlling a polarizing
state of said liquid-crystal device for each beam exiting said
input optical fibers.
20. The variable optical attenuator according to claim 1, wherein
said liquid-crystal device has a plurality of sets, each set
comprising liquid crystal and electrodes to be used for applying an
electric field to said liquid crystal, for controlling polarizing
states of the liquid-crystal device for different respective
polarizing components of said input light separated by said
birefringent device.
21. The variable optical attenuator according to claim 1, wherein
said liquid-crystal device is formed from liquid-crystal molecules
and glass plates to be used for sandwiching said liquid-crystal
molecules, and said reflection device is formed on the surface of
one of said glass plates.
22. A variable optical attenuator comprising: an input optical
system to which a plurality of input optical fibers are connected
and which has a plurality of input lenses that take beams exiting
said input optical fibers as input beams; a first birefringent
device provided on an output side of said input optical system; a
liquid-crystal device capable of varying the polarizing states of
respective input beams exiting said first birefringent device; a
second birefringent device provided on an output side of said
liquid-crystal device; and an output optical system to which a
plurality of output optical fibers are connected and which has a
plurality of output lenses for gathering output light exiting said
second birefringent device and coupling the gathered output light
to a corresponding output optical fiber.
23. The variable optical attenuator according to claim 22, wherein
said input optical system, said first liquid-crystal device, said
liquid-crystal device, said second birefringent device, and said
output optical system are integrated together.
24. The variable optical attenuator according to claim 23, wherein
said input optical system comprises an input fiber array block in
which a plurality of said input optical fibers are arranged and
connected in the form of an array; and an input lens array block in
which a plurality of said input lenses are arranged in the form of
an array according to the arrangement of said input optical fibers
provided in said input fiber array block; and wherein said output
optical system comprises an output fiber array block in which a
plurality of said output optical fibers are arranged and connected
in the form of an array; and an output lens array block in which a
plurality of said output lenses are arranged in the form of an
array according to the arrangement of said output optical fibers
provided in said output fiber array block.
25. The variable optical attenuator according to claim 22, wherein
said input optical system comprises an input fiber array block in
which a plurality of said input optical fibers are arranged and
connected in the form of an array; and an input lens array block in
which a plurality of said input lenses are arranged in the form of
an array according to the arrangement of said input optical fibers
provided in said input fiber array block; and wherein said output
optical system comprises an output fiber array block in which a
plurality of said output optical fibers are arranged and connected
in the form of an array; and an output lens array block in which a
plurality of said output lenses are arranged in the form of an
array according to the arrangement of said output optical fibers
provided in said output fiber array block.
26. The variable optical attenuator according to claim 22, wherein
said liquid-crystal device has a plurality of sets, each set
comprising liquid crystal and electrodes to be used for applying an
electric field to said liquid crystal, for controlling a polarizing
state of light exiting from said input optical fibers on a per-beam
basis.
27. The variable optical attenuator according to claim 22, wherein
said liquid-crystal device has a plurality of sets, each set
comprising liquid crystal and electrodes to be used for applying an
electric field to said liquid crystal, for controlling polarizing
states of different polarizing components of said input light
separated by said first birefringent device on a
per-polarizing-component basis.
Description
BACKGROUND OF THE INVENTION
[0001] (1) Field of the Invention
[0002] The present invention relates to a variable optical
attenuator, and more particularly, to a variable optical attenuator
capable of changing output optical power by means of varying the
magnitude of optical coupling existing between input and output
optical fibers through control of the polarizing state of
light.
[0003] (2) Description of the Related Art
[0004] In association with an increase in the traffic over the
Internet, the need to increase the capacity of optical
communication has recently become urgent. One of the measures for
increasing the capacity of optical communication is to increase a
bit rate, and another measure is to employ wavelength division
multiplexing (WDM). Prompt realization of an optical device which
constitutes such a system is desired.
[0005] Here, WDM transmission is a technique for transmitting a
plurality of wavelengths over a single optical transmission line
(e.g., an optical fiber), wherein data are transferred at
respective wavelengths, to thereby increase the capacity of
communication. However, when data are transmitted through the
optical fiber, propagation loss differs from one wavelength to
another, and after transmission over a long distance changes arise
in optical levels of the respective wavelengths.
[0006] When a branch device or an erbium-doped fiber (EDF)
amplifier is used in the optical transmission line, this phenomenon
becomes more noticeable. For this reason, optical levels at
respective wavelengths must be made constant before optical
transmission is performed. A solution for this is a technique
(called "pre-emphasis") for controlling an optical output achieved
at the time of transmission beforehand such that an optical level
achieved after transmission becomes constant, through use of a
variable optical attenuator (hereinafter also called an "optical
attenuator"), or the like, which controls levels of individual
wavelengths. However, under the assumption that WDM transmission
would be performed, optical levels must be set for respective
wavelengths (channels). Hence, there must be provided an optical
attenuator capable of varying optical power on a per-channel
basis.
[0007] However, under present circumstance, there are many cases
where optical attenuators are provided on a per-channel basis,
thereby rendering devices, such as optical repeaters, bulky and
incurring a cost hike. A technique described in Patent Publication
1 has hitherto been proposed as a measure for making the device
compact. Specifically, as shown in FIGS. 16A and 16B, development
has been pursued to constitute, as a single device, an optical
attenuator capable of varying individual optical power levels of a
plurality of channels through use of an optical waveguide device of
planar type (or a planar lightwave circuit: PLC) 100. FIG. 16A is a
top view of the optical attenuator, and FIG. 16B is a side view of
the optical attenuator.
[0008] In the optical attenuator shown in FIGS. 16A and 16B, tape
fibers (each being formed into a tape by stranding a plurality of
optical fibers) 200 are connected to mutually-opposing input and
output sections of the PLC 100 within a package (housing) 400. A
desired voltage is applied, by way of electrical terminals 300, to
electrodes provided in equal number to channels within the PCL 100,
thereby changing the refractive index of a waveguide on a
per-channel basis in order to change optical power.
[0009] Patent Publication 2 describes a conventional "handwritten
input display device" which enables handwritten input and display
of an image and a character by means of utilizing a phenomenon of
changing a polarizing state of light through control of arrangement
of liquid-crystal molecules.
[0010] [Patent Publication 1]
[0011] JP-A-2000-180803
[0012] [Patent Publication 2]
[0013] JP-A-63-201815
[0014] However, the above-described planar lightwave device 100
usually requires micromachining of a quartz substrate through
reactive ion etching (RIE) or like processing, thus incurring
costs. Further, sufficient miniaturization of the lightwave device
cannot be said to have been achieved, for reasons of a limitation
on the micromachining technique.
SUMMARY OF THE INVENTION
[0015] The invention has been conceived in view of the problem and
aims at providing a variable optical attenuator which is more
compact and less expensive than a conventional variable optical
attenuator.
[0016] To achieve the object, the variable optical attenuator of
the invention is characterized by comprising the following
elements.
[0017] (1) an input/output optical system to which are connected a
plurality of input optical fibers and a plurality of output optical
fibers and which has a plurality of input lenses for taking beams
having entered by way of the input optical fibers as input beams
and a plurality of output lenses for gathering output beams to be
coupled to the output optical fibers, to thereby couple the output
beams to the output optical fibers;
[0018] (2) a birefringent device provided on an output side of the
input/output optical system;
[0019] (3) a liquid crystal device capable of changing polarizing
states of the input beams exiting the birefringent device; and
[0020] (4) a reflection device which reflects beams passing through
the liquid-crystal device so as to return the beams to the output
lens of the input/output optical system by way of the
liquid-crystal device and the birefringent device.
[0021] Here, the input/output optical system, the birefringent
device, the liquid-crystal device, and the reflection device are
preferably integrated together.
[0022] The input/output optical system preferably comprises a fiber
array block, in which a plurality of the input optical fibers are
arranged and connected in the form of an array and a plurality of
the output optical fibers are arranged and connected in the form of
an array and in the same direction as that in which the input
optical fibers are arranged; and a lens array block, in which a
plurality of the input lenses are arranged in the form of an array
in accordance with the arrangement of the input optical fibers in
the input array fiber block and in which a plurality of the output
lenses are arranged in the form of an array in accordance with the
arrangement of the output optical fibers in the output array fiber
block.
[0023] The liquid-crystal device may preferably have a plurality of
sets, each set comprising liquid crystal and electrodes to be used
for applying an electric field to the liquid crystal, for
controlling polarizing states of different polarizing components of
the input light separated by the birefringent device on a
per-polarizing-component basis.
[0024] A variable optical attenuator according to another
embodiment of the invention has the following devices:
[0025] (1) an input optical system to which a plurality of input
optical fibers are connected and which has a plurality of input
lenses taking beams exiting from the input optical fibers as input
beams;
[0026] (2) a first birefringent device provided on an output side
of the input optical system;
[0027] (3) a liquid-crystal device capable of varying polarizing
state of input beams exiting the first birefringent device;
[0028] (4) a second birefringent device provided on an output side
of the liquid-crystal device; and
[0029] (5) an output optical system to which a plurality of output
optical fibers are connected and which has a plurality of output
lenses for gathering output light exiting the second birefringent
device and coupling the gathered output light to an output optical
fiber.
[0030] The variable optical attenuator of the invention yields the
following advantages:
[0031] (1) Input beams are caused to reciprocally pass through the
birefringent device and the liquid-crystal device between a
plurality of input optical fibers and a plurality of output optical
fibers, both being connected to the input/output optical system,
through use of the reflection device. Polarizing states of the
respective input beams are controlled by means of the
liquid-crystal device. The quantity of light coupled to the output
optical fiber can be changed freely for respective input beams;
that is, on a per-channel basis. A variable optical attenuator
compatible with multiple channels can be realized in the form of a
compact and inexpensive variable optical attenuator while
suppressing an increase in the size of the attenuator and an
increase in the area occupied by the attenuator, which would
otherwise be caused if the number of channels were increased.
[0032] (2) Here, if the input/output optical system, the
birefringent device, the liquid-crystal device, and the reflection
device are integrated together, the variable optical attenuator can
be made much more compact.
[0033] (3) Under the assumption that the respective input optical
fibers and the respective output optical fibers are arranged and
connected in the form of an array by means of a fiber array block
and that the respective input and output lenses are arranged in the
form of an array according to the arrangement of the optical fibers
by means of the lens array block, even when the number of channels
has been increased, the attenuator can be collectively configured
by forming individual devices into an array. Hence, the cost of the
optical attenuator array per channel can be significantly reduced
as compared with the related-art optical attenuator array, by means
of significantly curtailing the number of components.
[0034] (4) Further, if a pitch between the input optical fibers and
a pitch between the output optical fibers are set so as to become
greater than a pitch between the input lenses and a pitch between
the output lenses, an improvement in polarization extinction ratio
can be expected. Hence, occurrence of interference between channels
can be inhibited.
[0035] (5) Under the assumption that the reflection device is
formed from a coupler film which permits transmission of a portion
of the light exiting the liquid-crystal device and that an input
light monitor light-receiving unit for receiving the light having
passed through the coupler film is provided on the surface of the
coupler film. The power of input light can be monitored, and hence
there can be realized a compact, inexpensive variable optical
attenuator capable of incorporating an optical monitor function
that is indispensable as an optical output variable component.
[0036] (6) Under the assumption that there is further provided an
output light monitor light-receiving unit for receiving the light
not coupled to the output optical fiber as a result of a variation
in the polarizing state of the liquid-crystal device from among the
beams reflected from the reflection device, the quantity of light
attenuation can be monitored. Similarly, there can be realized a
compact, inexpensive variable optical attenuator capable of
incorporating an optical monitor function that is indispensable as
an optical output variable component.
[0037] (7) Under that assumption that, in order to control the
polarizing states of the liquid-crystal device for each beam
exiting the input optical fiber or for different respective
polarizing components of the input light separated by the
birefringent device, the liquid-crystal device is constituted by
comprising a plurality of sets, each set consisting of a piece of
liquid crystal and electrodes to be used for applying an electric
field to the liquid crystal, the polarizing state of the
liquid-crystal device can be controlled on a per-channel basis or
for respective polarizing components of different channels, the
quantity of light attenuation can be controlled more precisely, and
hence an improvement in polarization extinction ratio can be
expected.
[0038] (8) Further, under the assumption that the liquid-crystal
device is formed by comprising liquid-crystal molecules and glass
plates to be used for sandwiching the liquid-crystal molecules, and
the reflection device is formed on the surface of one of the glass
plates, the liquid-crystal device and the reflection device can be
integrated together, and hence the variable optical attenuator can
be downsized further.
[0039] (9) Under the assumption that a prism unit--which reflects a
portion of incident light in a direction crossing the direction of
an optical axis--is interposed between the fiber array block and
the lens array block and that a light-receiving unit for monitoring
input and output light which receives the light reflected from the
prism unit is provided, the power of input light and/or output
light can be monitored. Even in this case, there can be realized a
compact, inexpensive variable optical attenuator capable of
incorporating an optical monitor function that is indispensable as
an optical output variable component.
[0040] (10) Further, under the assumption that the light-receiving
unit is formed from a photodiode array--in which a plurality of
photodiodes, each photodiode having a P electrode on one surface
thereof and an N electrode on the other surface thereof, are
arranged in an array pattern on a conductive transparent substrate
such that the other surfaces come into contact with the transparent
substrate--and that a common terminal of the N electrodes of the
respective photodiodes are provided on the transparent substrate,
there is no necessity for providing an N electrode terminal on a
per-N-electrode basis. Hence, the number of wiring units is
curtailed, thereby improving efficiency. An attempt can be made to
downsize the variable optical attenuator by a great extent.
[0041] (11) Under the assumption that the light-receiving unit is
formed from a photodiode array, in which a plurality of
photodiodes, each having a P electrode on one surface thereof and
an N electrode formed around the P electrodes, are arranged in the
form of an array on a transparent substrate, a limitation imposed
on the materials which can be used for the transparent substrate
are mitigated, thereby broadening the range of choice of materials.
Therefore, the variable optical attenuator can be made further
inexpensive.
[0042] (12) Even when the input optical system and the output
optical system are constituted individually without use of a
reflection device, the variable optical attenuator enables a free
change in the amount of light coupled to the output optical fiber
on a per-channel basis. Hence, the variable optical attenuator can
be realized less expensively than a conventional variable optical
attenuator.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] FIG. 1A is a schematic plan view showing the basic
configuration of a variable optical attenuator employed as a first
embodiment of the invention in conjunction with a lightwave;
[0044] FIG. 1B is a schematic side view of the variable optical
attenuator shown in FIG. 1A;
[0045] FIG. 2 is a schematic perspective view showing the variable
optical attenuator shown in FIGS. 1A and 1B with portions of the
attenuator being made transparent;
[0046] FIG. 3 is a schematic diagram for describing the principle
on which a liquid-crystal element of the embodiment operates;
[0047] FIG. 4 is a schematic diagram for describing the principle
on which a liquid-crystal element of the embodiment operates;
[0048] FIG. 5 is a schematic diagram for describing the principle
on which a liquid-crystal element of the embodiment operates;
[0049] FIG. 6 is a schematic diagram for describing the principle
on which a liquid-crystal element of the embodiment operates;
[0050] FIG. 7A is a schematic plan view showing the configuration
of the principal section of the liquid-crystal element of the
embodiment;
[0051] FIG. 7B is a side view of the principal section when viewed
in the direction A shown in FIG. 7A;
[0052] FIG. 8 is a schematic plan view showing the configuration of
a variable optical attenuator array for describing a specific
example of the variable optical attenuator of the embodiment;
[0053] FIG. 9A is a schematic top view showing an example overview
of a variable optical attenuator array of the embodiment;
[0054] FIG. 9B is a schematic side view showing an example overview
of a variable optical attenuator array of the embodiment;
[0055] FIG. 10 is a schematic plan view showing a first
modification of the variable optical attenuator array of the
embodiment;
[0056] FIG. 11 is a schematic plan view showing a second
modification of the variable optical attenuator array of the
embodiment;
[0057] FIG. 12 is a schematic side view showing a third
modification of the variable optical attenuator array of the
embodiment;
[0058] FIGS. 13A to 13C are views for describing a first
configuration of a photodiode (PD) according to any of the
embodiments;
[0059] FIGS. 14A to 14C are views for describing a second
configuration of a photodiode (PD) according to any of the
embodiments;
[0060] FIG. 15 is a schematic plan view showing the basic
configuration of a variable optical attenuator employed as a second
embodiment of the invention in conjunction with an optical
path;
[0061] FIG. 16A is a schematic plan view showing the configuration
of a variable optical attenuator using a related-art planar
lightwave circuit (PLC); and
[0062] FIG. 16B is a schematic side view of the variable optical
attenuator shown in FIG. 16A.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0063] Embodiments of the invention will be described hereinbelow
by reference to the drawings.
[A] DESCRIPTION OF THE FIRST EMBODIMENT
(A1) Description of the Basic Configuration
[0064] FIG. 1A is a schematic plan view showing the basic
configuration of a variable optical attenuator (hereinafter also
called an "optical attenuator") according to a first embodiment of
the invention, along with a lightwave; FIG. 1B is a schematic side
view of the variable optical attenuator shown in FIG. 1A; and FIG.
2 is a schematic perspective view showing the variable optical
attenuator shown in FIGS. 1A and 1B with portions of the attenuator
being made transparent.
[0065] As shown in FIGS. 1A, 1B, and 2, the optical attenuator of
the embodiment is basically constituted of a fiber array block (a
fiber-arrayed precision device) 2, a lens array block (a
lens-arrayed precision device) 3, a birefringent crystal 4, a
liquid-crystal element (a liquid-crystal device) 5, and a
reflection element (reflection device) 6. The fiber array block 2,
the lens array block 3, the birefringent crystal 4, the
liquid-crystal element 5, and the reflection element 6 are
integrally arranged without any space therebetween such that input
planes of light and output planes of light remain in contact with
each other.
[0066] Here, an input light fiber 1a and an output light fiber 1b
are connected to the fiber array block (hereinafter also called
merely "fiber block") 2 in the same direction (e.g., the direction
of the Z axis shown in FIG. 1B). An input lens 3a and an output
lens 3b, which are arranged in the direction of the Z axis such
that the optical axes of the lenses are aligned with the optical
axes of the respective optical fibers 1a, 1b, are provided on the
lens array block (hereinafter also called simply a "lens block") 3.
A collimator lens or a light-gathering lens, which converts input
light into collimated light, can be employed as the input lens 3a
and the output lens 3b.
[0067] The fiber block 2 is also equipped with an input waveguide
(input port) 2a for causing the light originating from the core of
the input optical fiber 1a to propagate to and enter the input lens
3a of the lens block 3, and an output waveguide (output port) 2b
for causing the light originating from the output lens 3b to
propagate to and enter the core of the output optical fiber 1b.
[0068] Specifically, the fiber block 2 and the lens block 3
constitute an input/output optical system. In the lens block 3, the
input lens 3a performs the function of converging into collimated
light the light that has entered by way of the input port 2a. The
output lens 3b performs the function of gathering the light
reflected from the reflection element 6, which will be described
later, and coupling the thus-converged light to the output port 3b.
As shown in FIG. 1B, when a gap existing between the input lens 3a
and the output lens 3b (i.e., an input/output lens gap) G is taken
as 0.25 mm (=250 .mu.m), the input and output optical fibers 1a, 1b
are fixed such that a gap existing between the optical fibers in
the direction of the Z axis (i.e., an input/output fiber gap) "g"
assumes a value of about 0.3 mm (300 .mu.m).
[0069] A rutile plate (another crystal may also be usable) which is
cut so as to assume an optical axis at an angle of 45.degree., for
example, is used as the birefringent crystal (birefringent member)
4. As shown in FIGS. 1A and 2, if such a rutile plate is used, the
light that has entered by way of the input lens 3a will be
separated into polarized components (an ordinary beam 41 and an
extraordinary beam 42) (in the direction of the Y axis), which are
polarized orthogonal to each other, while propagating through the
rutile plate in the direction of the X axis. In FIG. 1A, the
thickness "d" of the rutile plate (i.e., the thickness of the
rutile plate in the direction of the X axis) is set to 2.5 mm such
that a distance S between the ordinary beam 41 and the
extraordinary beam 42 (i.e., a distance between points of
reflection in the direction of the Y axis on the reflection element
6), which are separated from each other, assumes a value of 0.25 mm
(250 .mu.m).
[0070] The liquid-crystal element 5 can change polarizing states of
the respective beams (beams) exiting the birefringent crystal 4
(i.e., for the normal beam 41 and the extraordinary beam 42,
respectively). The liquid-crystal element 5 has a structure in
which liquid crystal 53 is sandwiched between two glass plates 51,
52. There is utilized a phenomenon of a beam having passed through
the liquid-crystal element 5 being converted from a
linearly-polarized beam to an elliptically-polarized beam, by means
of application of an arbitrary electric field between the glass
plates 51, 52 so as to change the birefringence of the
liquid-crystal element 5. If such a phenomenon can be utilized, the
liquid-crystal element 5 may be a commonly-used liquid-crystal
element of nematic type or a liquid-crystal element of another type
(smectic type).
[0071] For instance, the structure of the liquid-crystal element 5
of a twisted nematic (TN) type will be described by reference to
"Principle of a Liquid-Crystal Display" (see the URL
http://www.sharp.co.jp/products/lc- d/tech/s2.sub.--1.html on the
Internet, Sharp Corporation). As schematically shown in FIGS. 3 and
4, the liquid-crystal element 5 has a structure in which molecules
53' of the liquid crystal 53 are sandwiched between the glass
plates (orientation films) 51, 52 having trenches engraved therein
in given directions while orientations of the trenches of the glass
plates are offset from each other by 90.degree..
[0072] By means of such a structure, molecules 53' of the liquid
crystal 53 (hereinafter denoted as "liquid-crystal molecules 53')
having a loose regularity in the direction of a major axis in a
natural state are arranged along the trenches of the respective
glass plates 51, 52. Further, the liquid-crystal molecules 53'
remaining in contact with the glass plate 51 and the liquid-crystal
molecules 53' remaining in contact with the glass plate 52 are
twisted from each other by 90.degree. between the glass plates 51,
52.
[0073] Light travels along a gap between the liquid-crystal
molecules 53'. Hence, when the arrangements of the liquid-crystal
molecules 53' are twisted, and the light also travels along a
twisted path, as schematically shown in FIG. 5 (i.e., a
linearly-polarized beam is converted into an elliptically-polarized
beam). As schematically shown in FIG. 6, when a voltage is applied
between the glass plates 51, 52, the arrangement of the
liquid-crystal molecules 53' is changed (i.e., aligned along the
electric field) in accordance with the voltage. Hence, light
travels in straight lines (i.e., a linearly-polarized beam travels
in unmodified form).
[0074] On the basis of the above-described principle, the
liquid-crystal element 5 can consecutively change the polarizing
state of an input beam in accordance with a voltage (i.e., an
electric field) applied from the outside. Here, in order to
independently change (control) the polarizing state of the ordinary
beam 41 and that of the extraordinary beam 42 on a per-beam basis
in the same manner as mentioned previously, the liquid-crystal
element 5 is configured in, e.g., a manner shown in FIGS. 7A and
7B.
[0075] FIG. 7A is a schematic plan view showing the configuration
of the principal section of the liquid-crystal element 5 of the
embodiment; and FIG. 7B is a side view of the principal section
when viewed in the direction A shown in FIG. 7A. As shown in FIGS.
7A and 7B, the liquid-crystal 53 partitioned by sealing material 54
constitutes a set in conjunction with transparent (translucent)
electrodes 55a, 55b to be used for applying a voltage (electric
field) to the liquid-crystal 53. The set is arranged between the
glass plates 51, 52 for the ordinary beam 41 and the extraordinary
beam 42 (i.e., for different respective polarization components)
independently. For example, an indium-tin oxide (ITO) electrode can
be used for the transparent electrodes 55a, 55b.
[0076] However, the set consisting of the liquid crystal 53 and the
transparent electrodes 55a, 55b is not necessarily provided for the
ordinary beam 41 and the extraordinary beam 42, respectively. It
may be the case that only sets equal in number to input
beams--which are not yet separated from each other (i.e., input
ports)--are provided as common sets for the ordinary beam 41 and
the extraordinary beam 42. However, providing separate sets for the
ordinary beam 41 and the extraordinary beam 42 is preferable,
because the quantity of light attenuation can be controlled more
precisely. Hence, an improvement in polarization extinction ratio
can be expected.
[0077] The reflection element 6 reflects the light having passed
through the liquid-crystal element 5, to thereby introduce the
light again into the liquid-crystal element 5 and the birefringent
crystal 4. In the embodiment, the reflection element is formed as a
total reflection film formed on the plane of light exit of the
liquid-crystal element 5 (i.e., the back of the glass plate 52).
The total reflection film may be a multilayer dielectric film or a
metal film (Al, Au or the like). Here, the reflection element 6 may
be provided as an individual device on a stage subsequent to the
liquid-crystal element 5. As mentioned above, integrating the
reflection element 6 with the liquid-crystal element 5 through
formation of a reflection film is advantageous for miniaturization
of a variable optical attenuator.
[0078] The basic operation of the optical attenuator of the
embodiment having the foregoing configuration will now be
described. First, the light exiting the upper input optical fiber
1a enters the input lens 3a provided in the direction of the
optical axis after having passed through the input port 2a, as well
as into the birefringent crystal 4 after having been converted into
collimated light by the input lens 3a.
[0079] The light having entered the birefringent crystal 4 is
divided into the ordinary beam 41 and the extraordinary beam 42,
and the thus-divided beams enter the liquid-crystal element 5. The
liquid-crystal element 5 is provided with the pieces of liquid
crystal 53 and the transparent electrodes 55a, 55b, which are
provided for the respective beams as mentioned previously. The
pieces of liquid crystal 53 and the transparent electrodes 55a, 55b
can be controlled independently. Hence, the polarizing state of the
ordinary beam 41 and that of the extraordinary beam 42, both beams
having entered the liquid-crystal element 5, are independently
controlled by the corresponding pieces of liquid crystal 53.
[0080] As a result, the light having passed through the
liquid-crystal element 5 is converted from, e.g.,
linearly-polarized light into elliptically-polarized light (i.e., a
state in which the linearly-polarized light component is merged
with a vertically-polarized light component), by means of
birefringence of the liquid crystal 53, and enters the reflection
element 6 formed on the back of the liquid-crystal element 5.
[0081] The light reflected from the reflection element 6 again
enters the liquid-crystal element 5. By means of birefringence of a
corresponding piece of liquid crystal 53, a change similar to that
mentioned previously arises in the polarizing state of light, and
the light enters the birefringent crystal 4. Of the beams having
entered the birefringent crystal 4, only a component which is
identical in polarizing state with the light having entered the
birefringent crystal 4 by way of the input lens 3 is finally
coupled with the lower output port 2b by way of the output lens 3b.
The light is then output to the output optical fiber 1b. As shown
in FIG. 1A, other components (beams) 43, 44 do not return to and
are not coupled with the output port 2b.
[0082] Therefore, the arrangement of the liquid-crystal molecules
53' is controlled through control of the voltage applied to the two
electrodes 55a, 55b provided for the respective pieces of liquid
crystal 53. Thereby, the polarizing state of the light that travels
back and forth within the birefringent crystal 4 and passes through
the liquid-crystal element 5 is controlled for each beam input to
the liquid-crystal element 5. As a result, the quantity of light
coupled to the output port 2b (i.e., the output optical fiber 1b)
can be changed freely on a per-channel basis. Thus, the optical
output power can be changed on a per-channel basis.
(A2) Description of a Specific Example
[0083] A variable optical attenuator array will now be described
hereinbelow as a specific example of the invention on the premise
that the array has the foregoing basic configuration.
[0084] FIG. 8 is a schematic top view showing the configuration of
a variable optical attenuator array of the embodiment. The variable
optical attenuator array shown in FIG. 8 has a structure in which a
multicore tape fiber 10 (including 12 cores)--into which a
plurality of input optical fibers 1a (twelve input optical fibers
in FIG. 8) are aggregated in the form of a tape--is connected to an
upper layer section of the fiber block 2 as an input tape
fiber.
[0085] Although not shown in FIG. 8, an analogous multicore tape
fiber (including twelve cores) is connected to a lower layer
section of the fiber block 2 as an output tape fiber. Specifically,
in the present embodiment, the tape fibers are fixed to the fiber
block 2 so as to be stacked on top of each other in two layers in a
vertical direction (i.e., a direction identical with the direction
of the Z axis shown in FIG. 2) with desired accuracy. An
epoxy-based optical adhesive or the like, for instance, is used for
fixing the multicore tape fibers (hereinafter also called simply
"tape fibers").
[0086] The input ports 2a--which are equal in number with the cores
of the tape fiber 10 (i.e., twelve input ports)--are arranged into
an array within an X-Y plane of the upper layer section of the
fiber block 2 at an interval between fiber cores of the input tape
fiber 10 (e.g., a pitch of 250 .mu.m). Similarly, the twelve output
ports 2b are arranged into an array within the X-Y plane of the
lower layer section at the pitch between the fiber cores.
[0087] Twelve input lenses 3a are arranged within the X-Y plane of
an upper layer section of the lens block 3 so as to coincide with
the optical axes of the respective input ports 2a. Twelve output
lenses 3b are arranged within the X-Y plane of a lower layer
section of the lens block 3 so as to coincide with the optical axes
of the respective output ports 2b.
[0088] Specifically, a total of 24 (2.times.12) ports are arranged
into an array within a Y-Z plane in the fiber block 2. Similarly, a
total of 24 (2.times.12) lenses are arranged into an array within
the Y-Z plane in the lens block 3 in agreement with the arrangement
of the ports in the fiber block 2 (i.e., the arrangement of the
input and output optical fibers 1a, 1b).
[0089] The thickness "d" of the birefringent crystal 4 is set to 1
mm such that a distance S between the ordinary beam 41 and the
extraordinary beam 42 assumes a value of about 0.1 mm (100
.mu.m).
[0090] As mentioned previously by reference to FIGS. 7A and 7B, the
set consisting of the liquid crystal 53 and the transparent
electrodes 55a, 55b, the liquid crystal being partitioned by the
sealing material 54, is provided in the liquid-crystal element 5
for the respective ordinary and extraordinary beams 41, 42 of the
light having entered by way of the respective input ports 2a (i.e.,
a total of 24 sets).
[0091] Even in this case, the only requirement for the
liquid-crystal element 5 is to use a single glass plate 51 (or 52).
The glass plate 52 can be readily formed into an array by means of
forming electrodes in one glass plate 52, each electrode having a
width corresponding to the size of a beam (about 200 .mu.m). The
set consisting of the liquid crystal 53 and the transparent
electrodes 55a, 55b may be provided for each input port so as to be
common to the ordinary beam 41 and the extraordinary beam 42.
[0092] As mentioned above, the variable optical attenuator array
compatible with multiple channels (12 channels) can be implemented
in the form of a compact, inexpensive variable optical attenuator
array while inhibiting an increase in the size of the array and the
area occupied by the same, which would otherwise be caused by an
increase in the number of channels. Even when the number of
channels has been increased, the attenuator can be collectively
configured by forming individual members into an array. Hence, the
price of the optical attenuator array per channel can be
significantly reduced when compared with the related-art optical
attenuator array.
[0093] In particular, the variable optical attenuator is formed as
a single piece by arranging the fiber block 2, the lens block 3,
the birefringent crystal 4, the liquid-crystal element 5, and the
reflection element 6 without any space therebetween. When compared
with a related-art attenuator using, e.g., a Faraday rotary, the
optical attenuator of the invention can be miniaturized
significantly.
[0094] FIGS. 9A and 9B show an example overview of a product of a
variable optical attenuator array of the embodiment. FIG. 9A is a
schematic top view showing an example overview of a product of a
variable optical attenuator array of the embodiment, and FIG. 9B is
a schematic side view showing an overview of the same product. As
shown in FIGS. 9A and 9B, the variable optical attenuator array is
constituted by the fiber block 2, the lens block 3, the
birefringent crystal 4, the liquid-crystal element 5, and the
reflection element 6, which are housed in a premolded package
(housing) 11 (having a length of about 18 mm, a width of about 8
mm, and a thickness of about 5 mm) made of resin such as
polyphenylenesulfide resin (PPS) or epoxy resin (alternatively, the
housing may be made of metal). In FIGS. 9A and 9B, reference
numeral 12 designates an electrical terminal, and a desired voltage
is applied to the transparent electrodes 55a, 55b of the
liquid-crystal element 5 by way of the electrical terminal 12.
[0095] If an optical system equivalent to that mentioned above can
be achieved, reducing the gap between the lenses in the direction
of the Y axis so as to become smaller than 250 .mu.m presents no
problem. As a matter of course, fixing of the tape fiber is not
limited solely to use of an adhesive. In lieu of separate tape
fibers being used for input and output purposes respectively, a
commonly available fiber having 2.times.12 cores can be used for
constituting the input/output optical system.
(A3) Description of a First Modification
[0096] FIG. 10 is a schematic plan view showing a first
modification of the previously-described variable optical
attenuator array. In contrast with the variable optical attenuator
shown in FIG. 8, in the variable optical attenuator shown in FIG.
10 the thickness of the birefringent crystal 4 (i.e., the length of
the crystal in the direction of the X axis) is set to about 2.5 mm
such that the distance S between the ordinary beam 41 and the
extraordinary beam 42, having been divided by the reflection
element 6, assumes a value of about 250 .mu.m, and the pitch
between the input ports 2a is set (to about 750 .mu.m) so as to
become greater than the pitch between the input lenses 3a (about
250 .mu.m). Therefore, in the variable optical attenuator shown in
FIG. 10, the number of input optical fibers 1a and the number of
input ports 2a (i.e., the number of channels) are set to "4."
[0097] Although omitted from FIG. 10, the input optical fibers 1a
and the output optical fibers 1b equal in number to the input ports
2a are arranged at a lower layer section of the fiber block 2 at
the same pitch as that existing between the input optical fibers 1a
and that existing between the input ports 2a, and the output lenses
3b equal in number to the input lenses 3a are provided in a lower
layer section of the lens block 3 at the same pitch as that
existing between the input lenses 3a.
[0098] As mentioned above, the pitch between the input optical
fibers 1a and that existing between the output optical fibers 1b
are set so as to become greater than the pitch existing between the
input lenses 3a and that existing between the output lenses 3b. As
a result, a large polarization extinction ratio can be ensured,
thereby inhibiting occurrence of interference between adjacent
ports (i.e., inter-channel interference).
[0099] Therefore, in this case, the liquid-crystal element 5 is
given such a size (e.g., 0.5 mm in the direction of the Y axis and
2.5 mm in the direction of Z axis) that all ports can be covered
with one set consisting of a piece of liquid crystal 53 and the
transparent electrodes 55a, 55b. The degree of light attenuation in
all channels (ports) can also be collectively controlled. Needless
to say, it is better to provide the set consisting of the liquid
crystal 53 and the transparent electrodes 55a, 55b for controlling
channels (for each of the ordinary beam 41 and the extraordinary
beam 42) separately, which arrangement can be expected to yield a
great improvement in control accuracy and polarization extinction
ratio.
[0100] Even in this embodiment, an optical fiber array (an
integrated optical fiber) may be used for the input optical fibers
1a (output optical fibers 1b) and the input lenses 3a (output
lenses 3b).
(A4) Description of a Second Modification
[0101] Next, FIG. 11 is a schematic plan view showing a second
modification of the previously-described variable optical
attenuator array. The variable optical attenuator shown in FIG. 11
is identical with that described by reference to FIG. 10. A
difference between the variable optical attenuator of this
embodiment and that shown in FIG. 10 lies in that the reflection
element 6 is constituted not as a total reflection film but as a
coupler film 6a for enabling passage of a portion of the incident
light; that photodiodes (PD) 61 for monitoring light are arranged
in an array in the direction of the Y axis at a position rearward
of the coupler film 6a; and that PD blocks 30, each consisting of
two light monitor PDs 31, 32, are provided on the surface of the
lens block 3 opposing the fiber block 2 such that the PD blocks 30
are provided on both sides of each input port 2a.
[0102] The PDs (light-receiving sections) 61 situated rearward of
the coupler film 6a are provided at least at positions where the
beam exiting the liquid-crystal element 5 (or the ordinary beam 41
and the extraordinary beam 42 separated by the birefringent crystal
4) arrives at the coupler film 6a. Each of the PDs 61 can monitor
the quantity of input light (i.e., the power of input light). The
PDs 61 may be arranged individually as discrete components.
However, in terms of a reduction in the number of components and a
reduction in the number of man-hours for manufacturing, use of a PD
device array--in which PDs are integrally arranged in an array in
agreement with a pitch between the arrival positions--is
preferable.
[0103] The pair of PDs (light-receiving sections) 31, 32 situated
in front of the lens block 3 are provided for receiving beams which
do not return to (or are not coupled to) the output port 2b from
among the beams reflected from the coupler film 6a. Here, for
example, the PD 31 is arranged so as to receive reflected light
(output light) of the extraordinary beam 42 which is not coupled
with the output port 2b. The remaining PD 32 is arranged so as to
receive reflected light (output light) of the ordinary beam 41,
which is not coupled to the output port 2b. Detailed configurations
of the PDs 31, 32, and 62 will be described later.
[0104] Operation of the variable optical attenuator array having
the foregoing configuration will now be described. The light
exiting the input optical fiber 1a enters a corresponding input
lens 3a by way of a corresponding input port 2a. The light is then
converted into collimated light by means of the input lens 3a, and
the thus-converted light enters the birefringent crystal 4. The
birefringent crystal 4 separates the input light into the ordinary
beam 41 and the extraordinary beam 42. The beams pass through the
liquid-crystal element 5 and enter the coupler film 6a.
[0105] The beams having passed through the coupler film 6a (i.e.,
the ordinary beam 41 and the extraordinary beam 42) enter the PDs
61. A PD current for the ordinary beam 41 and a PD current for the
extraordinary beam 42 are output. On the assumption that a PD
current value pertaining to the ordinary beam 41 is taken as PD1
and a PD current value pertaining to the extraordinary beam 42 is
taken as PD2, the sum of the two PD current values (i.e., the sum
of light-receiving sensitivities=PD1+PD2) corresponds to the power
of input light.
[0106] Of the beams reflected from the coupler film 6a, a beam
having the same polarizing component as that of the incident light
is coupled to the output port 2b by way of the birefringent crystal
4 in the manner mentioned previously. The beam that enters the
birefringent crystal 4 as a result of polarizing components of the
beam having been changed by the liquid-crystal element 5 is divided
into an ordinary beam and an extraordinary beam as in the case of
the beam traveling forward in the birefringent crystal. As a
result, there arise a beam returning to the output port 2b and
beams 43, 44 which undergo birefringence, to thus travel beside
both sides of the output ports 2b (i.e., positions separated from
both sides of the output port 2b by 250 .mu.m), and do not return
to the output port 2b.
[0107] The beams 43, 44 are received by the PDs 31, 32,
respectively. Here, provided that the PD current value of an
ordinary beam is taken as PD3 and the PD current value of an
extraordinary beam is taken as PD4, the sum of PD3 and PD4 (i.e., a
PD output value) corresponds to the quantity of light which has not
coupled with the output port 2b. Therefore, a value determined by
subtracting the PD output value (i.e., the sum of PD3 and PD4)
pertaining to the output light from the PD output value (i.e., the
sum of PD1 and PD2) pertaining to the input light corresponds to
the quantity of light attenuation.
[0108] By means of calculation of the PD output values, the power
of input light and that of output light can be monitored. There can
be realized a compact, inexpensive variable optical attenuator
capable of incorporating an optical monitor function that is
indispensable as an optical output variable component.
[0109] Use of a PD of back incidence type--which enables direct
adhesion of the coupler film 6a and the lens array block 3 (or the
birefringent crystal 4) as structures of the PDs 31, 32, and 61--is
preferable. As a matter of course, a commonly-employed PD of front
incidence type can also be used. However, in this case, a required
space must be provided between the coupler film 6a and the light
incidence surface of PDs, in view of convenience of wiring. For
instance, an epoxy-based optical adhesive is preferable for fixing
PDs.
[0110] A preferable light-receiving diameter of the PDs 31, 32, and
61 is, e.g., 300 .mu.m, regardless of the types of PDs employed. In
the case of a PD of front incidence type, a PD having a smaller
light-receiving diameter can also be applied to the PDs by means of
reducing the diameter of a beam through arrangement of lenses in
the space.
(A5) Description of a Third Modification
[0111] FIG. 12 is a schematic side view showing a third
modification of the previously-described variable optical
attenuator array. The variable optical attenuator shown in FIG. 12
is identical with that shown in FIG. 10. A difference between the
variable optical attenuator of this embodiment and that shown in
FIG. 10 lies in that a prism (coupler film prism) 13 formed from
sandwiched coupler films 13a, 13b is provided between the fiber
block 2 and the lens block 3 such that input light and output light
can be extracted in a direction orthogonal to the direction of the
optical axis (i.e., the direction of the X axis); that PDs (input
monitor PDs: light-receiving sections) 14a are provided on the
upper surface of the prism 13 at positions corresponding to the
respective input ports 2a; and that PDs (output monitor PDs:
light-receiving sections) 14b are provided on a lower surface of
the prism 13 at positions corresponding to the respective output
ports 2b.
[0112] Here, the coupler films 13a, 13b have characteristics such
that the film reflects a portion of incident light (e.g., 5% of
incident light) in a direction orthogonal to the direction of the
optical axis and that the film allows passage of the remaining
portion (95%) through the coupler films in unmodified form.
Consequently, the coupler film 13a reflects 5% of the light having
entered by way of the input port 2a, to thereby cause the light to
enter the PDs 14a, and allows passage of the remaining 95% of the
light, to thereby cause the light to enter corresponding input
lenses 3a of the lens block 3.
[0113] The coupler film 13b reflects 5% of the light exiting the
output lens 3b of the lens block 3, to thereby cause the light to
enter the PD 14b and allows passage of the remaining 95% of the
light, to thereby cause the remaining light to enter corresponding
output ports 2b. The thickness of the prism 13 (i.e., the length of
the prism 13 in the direction of the X axis) is set to a value of,
e.g., 500 .mu.m. The transmission factor (a reflection factor) of
the coupler films 13a, 13b can be changed, as required.
[0114] As a result, even the variable optical attenuator of the
embodiment can also monitor the power of input light and the power
of output light on a per-channel basis by means of the PDs 14a,
14b. Hence, the optical monitor function that is indispensable for
a variable optical output component can be incorporated into the
optical attenuator while an attempt is made to attain
miniaturization and cost cutting.
[0115] The PDs 14a, 14b are also preferably formed by causing PDs
of back incidence types to adhere directly to the surface of the
coupler film prism 13. Use of an epoxy-based adhesive for fixing
the PDs is preferable. As a matter of course, even in this case, a
commonly-employed PD of front incidence type can also be used. In
terms of convenience of wiring, there cannot be adopted a
configuration in which the PDs are caused to adhere directly on the
surface of the prism 13. Hence, a required space must be
provided.
[0116] A preferable light-receiving diameter of the PDs 14a, 14b
is, e.g., 300 .mu.m, regardless of the types of PDs employed. In
the case of a PD of front incidence type, a PD having a smaller
light-receiving diameter can also be applied to the PDs by means of
reducing the diameter of a beam through arrangement of lenses in
the space. The PDs 14a (14b) may be provided on the prism 13
discretely. However, in terms of a reduction in the number of
components and a reduction in the number of man-hours for
manufacturing, use of a PD device array--in which PDs are
integrally arranged in an array in agreement with a pitch between
the input ports 2a (or output ports 2b)--is advantageous.
[0117] The previously-described embodiment adopts a pair consisting
of the coupler film 13a and the input monitor PD 14a and another
pair consisting of the coupler film 13b and the output monitor PD
14b so that input and output light can be extracted and monitored
respectively. As a matter of course, it may be the case that only
one of the pairs is adopted.
(A6) Connection Pattern of PDs
[0118] The configuration of the previously-described PD 61 and
those of the PDs 31, 32, 14a, and 14b will be described in detail
hereinbelow. For the sake of convenience of description, these PDs
are not distinguished from each other and are denoted as PDs
20.
(A6.1) First configuration example of PD 20
[0119] FIGS. 13A, 13B, and 13C are views for describing the first
configuration example of the PD 20. FIG. 13A is a side view; FIG.
13B is a top view; and FIG. 13C is a view of the PD when observed
through the back of FIG. 13B.
[0120] The respective PDs 20 shown in FIGS. 13A, 13B, and 13C are
of back incidence type. In each of the PDs 20, a P electrode 22 is
provided on one surface, and an N electrode 23 is provided on the
other surface (light-receiving surface). The surface of the PD 20
provided with the N electrode 23 is taken as a mount surface, and
the PDs 20 are arranged and fixed on the conductive transparent
substrate 21, such as a transparent electrode, in the form of an
array. P electrode terminals 24 are connected to the respective P
electrodes 22, and a common terminal (N electrode common terminal)
25 is connected to the respective N electrodes 23 on the
transparent substrate 21.
[0121] Adoption of such a structure obviates a necessity for
providing N electrode terminals for the respective N electrodes 23,
thereby curtailing the number of wires and achieving improved
efficiency. Thus, an attempt can be made to pursue a more compact
and lower-cost variable optical attenuator.
(A6.2) Second configuration example of PD 20
[0122] FIGS. 14A, 14B, and 14C are views for describing the second
configuration example of the PD 20. FIG. 14A is a side view; FIG.
14B is a top view; and FIG. 14C is a view of the PD when observed
through the back of FIG. 14B. The respective PDs 20 shown in FIGS.
14A, 14B, and 14C are also of back incidence type. In this case,
the PD 20 has the following structure. Namely, one surface (light
incidence surface) of the PD 20 is taken as a mount surface, and
the PDs 20 are arranged on a transparent substrate 21' in the form
of an array. The Pelectrode 22 connected to the P electrode
terminal 24 and the N electrode terminal 26 provided around the P
electrode terminal are provided on the other surface.
[0123] Here, the transparent substrate 21' may possess conductivity
as in the case of the previously-described transparent substrate 20
or may be of non-conductive type. In this case, a limitation
imposed on materials which can be used for the transparent
substrate 21' is mitigated as compared with the first configuration
example, thereby broadening the range of choice of materials.
Therefore, an attempt can be made to curtail costs of the variable
optical attenuator to a great extent through selection of
material.
[0124] When such PDs 20 are provided, it is desirable to house the
PDs 20 in the premolded package 11 (see FIGS. 9A and 9B) while
sealing portions of the variable optical attenuator with resin so
as to cover wire portions of the terminals 24 (or 25) and 26.
[B] DESCRIPTION OF SECOND EMBODIMENT
[0125] Although in the first embodiment the variable optical
attenuator of reflection type is configured through use of the
reflection element 6, the variable optical attenuator can also be
configured without use of the reflection element 6 in the same
manner as in the first embodiment.
[0126] For instance, as shown in FIG. 15, a fiber (array) block
(fiber-arrayed precision device) 2A, a lens (array) block
(lens-arrayed precision device) 3A, and a birefringent crystal 4A
are provided on an input side of the liquid-crystal element 5; and
a fiber (array) block (fiber-arrayed precision device) 2B, a lens
(array) block (lens-arrayed precision device) 3B, and a
birefringent crystal 4B are provided on an output side of the
liquid-crystal element 5 such that the fiber blocks, the lens
blocks, and the birefringent crystals become symmetrical about the
center of the liquid-crystal element 5. Even in such a case, the
fiber blocks 2A, 2B, the lens blocks 3A, 3B, the birefringent
crystals 4A, 4B, and the liquid-crystal element 5 are arranged in
an integrated fashion without a space therebetween while light
input surfaces or light output surfaces respectively remain in
contact with each other.
[0127] Even in this embodiment, the input-side fiber block 2A is
provided with input ports 2a which are provided for each input
optical fiber 1a to be connected and cause the light exiting the
input port 1a to propagate through the lens block 3A. Input lenses
3a arranged in agreement with the arrangement of the input ports 2a
(in more detail, so as to coincide with optical axes of input light
exiting the input ports 2a) are provided in the input-side lens
block 3A.
[0128] The input-side birefringent crystal 4A and the output-side
birefringent crystal 4B (i.e., the first and second refractive
devices) are identical with or analogous to the birefringent
crystal 4 of the first embodiment. The liquid-crystal element 5 is
also identical with or analogous to that described in connection
with the first embodiment.
[0129] Output lenses 3b arranged so as to coincide with optical
axes of the input lenses 3a are provided in the output-side lens
block 3B. Output ports 2b--which are arranged so as to coincide
with optical axes of the input lenses 3a and cause the light
exiting corresponding output lenses 3b to propagate to the output
optical fibers 1b--are provided in the output-side fiber block
2B.
[0130] The configuration of this embodiment corresponds to a
configuration in which the fiber block 2, the lens block 3, and the
birefringent crystal 4, which are used in the input/output optical
system in the first embodiment for both forward and backward
directions, are provided separately for the input optical system
(i.e., the fiber block 2A, the lens block 3A, and the birefringent
crystal 4A) and the output optical system (i.e., the fiber block
2B, the lens block 3B, and the birefringent crystal 4B).
[0131] FIG. 15 shows only the pair of fibers 1a, 1b, the pair of
ports 2a, 2b, and the pair of input lenses 3a, 3b. As a matter of
course, those pairs are provided in equal number to required
channels as in the case of, e.g., the embodiments shown in FIGS. 8
and 10.
[0132] Operation of the optical attenuator of the embodiment having
the foregoing configuration will now be described. The light
exiting the input optical fiber 1a enters the input lens 3a
provided in the axial direction by way of the input port 2a. The
light is then converted into collimated light by means of the input
lens 3a, and the thus-converted light enters the input-side
birefringent crystal 4A.
[0133] The light having entered the birefringent crystal 4A is
divided into the ordinary beam 41 and the extraordinary beam 42,
and the thus-divided beams enter the liquid-crystal element 5. Even
in this embodiment, the liquid-crystal element 5 is equipped with
the liquid crystal 53 and the transparent electrodes 55a, 55b for
each beam, to thereby enable independent control of the beams.
Hence, the polarizing state of the ordinary beam 41 and that of the
extraordinary beam 42, both beams having entered the liquid crystal
element 5, are individually controlled by the liquid crystal 53.
Subsequently, the beams enter the output-side birefringent crystal
4B.
[0134] Of the beams having entered the birefringent crystal 4B,
only the light components whose polarizing states coincide with the
polarizing state of the light having entered the birefringent
crystal 4A by way of the input lens 3 (i.e., forward-traveling
light) are finally coupled to the output port 2b by way of the
output lenses 3b and output to the output optical fibers 1b.
[0135] Therefore, even in this case, the quantity of light coupled
to the output port 2b (output optical fibers 1b) can be freely
changed on a per-channel basis by means of control of a voltage
applied to the liquid-crystal element 5. Hence, the output power of
light can be changed on a per-channel basis, and the variable
optical attenuator can be realized at lower cost than can the
conventional variable optical attenuator.
[0136] Even in this case, the fiber blocks 2A, 2B; the lens blocks
3A, 3B; the birefringent crystals 4A, 4B; and the liquid-crystal
element 5 are arranged in an integrated fashion without a space
therebetween while light input surfaces or light output surfaces
remain in contact with each other. Hence, when compared with a
related-art attenuator using, e.g., a Faraday rotary, the optical
attenuator of the invention can be downsized significantly.
[0137] Even in this embodiment, there is no necessity for separate
provision of the set consisting of the liquid crystal 53 and the
transparent electrodes 55a, 55b for the ordinary beam 41 and the
set for the extraordinary beam 42. The sets maybe provided in equal
number to input beams before separation (i.e., the number of input
ports) so as to be shared between the ordinary beam 41 and the
extraordinary beam 42. When a polarization extinction ratio is
improved by increasing the pitch between the ports, one set
consisting of the liquid crystal 53 and the transparent electrodes
55a, 55b may cover the entire port.
[0138] As in the case of the embodiment described by reference to
FIG. 12, a prism having a coupler film (i.e., a coupler film prism)
may be provided so as to extract input and output beams in a
direction orthogonal to the direction of the optical axis (i.e.,
the direction of the X axis), and monitor PDs for receiving the
thus-extracted beams may also be provided. By means of such a
configuration, even this embodiment enables monitoring of power of
input and/or output light, and hence a light monitor function
indispensable for a light output variation component can be
incorporated into the optical attenuator.
[0139] Needless to say, the invention is not limited to the
foregoing embodiments and can be implemented while being modified
in various manners within the scope of the invention.
* * * * *
References